The present invention relates to a mechanical pencil. More particularly this invention concerns such a pencil with automatic lead advance.
A mechanical pencil is known having a lead-protecting sleeve which surrounds the portion of the lead projecting from the tip of the pencil. This sleeve serves to support a relatively thin lead, by which is meant here any rod or stick of writing material used by the mechanical pencil, and prevents it from breaking. As the lead wears away the protecting sleeve automatically slides back into the body of the pencil. When insufficient lead is projecting from the tip, even with a fully retracted lead-protecting sleeve, the user actuates the lead-advance button at the rear end of the pencil so as to incrementally advance the lead another step. Thus writing can continue with the newly-exposed portion of lead, around which the protecting sleeve has also automatically been advanced.
It has been suggested to combine this protecting-sleeve structure with an automatic lead-feed mechanism inside the pencil. In such an arrangement the protecting sleeve is spring-biased forwardly, that is away from the rear or eraser end of the pencil and toward the writing tip thereof. The lead projecting from the pencil tip is held in a standard slitted collet-like holder which is spring biased not into a position clamping the lead as is usual, but into a position allowing displacement of the lead relative to the holder. This holder is tapered away from the tip of the pencil and is limitedly longitudinally displaceable in the pencil. Thus when the tip of the pencil is pressed against the paper the lead is pushed backwardly, frictionally engaging the holder and forcing it into the seat so as tightly to clamp the lead in place. When this backwardly effective writing pressure is let up the arrangement automatically steps the lead forward a tiny increment.
The main difficulty with such an arrangement is that the backward displacement of the lead each time it is brought into contact with the writing paper or the like is relatively great. Thus each time the pencil is brought into contact with the paper the lead must be depressed a certain distance before writing can commence. Such action interferes with neat writing and, indeed, makes it impossible for some persons to write legibly with such a pencil. Another disadvantage with this type of arrangement is that it is necessary, in case the lead breaks or a new lead must be fed to the tip, to repeatedly press the tip of the pencil against the page in order to feed a new lead to it. Due to the smallness of the increments by which the lead is advanced during such feed, it is therefore necessary for the pencil user to press down many times in order to feed a new lead to the tip.